Display device having liquid crystal layer and switchable optical layer

ABSTRACT

A display device including a first electrode, a liquid crystal layer positioned under and connected to the first electrode, a second electrode and a switchable optical layer. The switchable optical layer includes either a transparent state or a non-transparent state and is electrically switchable between the transparent state and the non-transparent state. The non-transparent state is a reflective state or a selectively emissive state. The switchable optical layer is positioned above and connected to the second electrode. A third electrode is positioned between the liquid crystal layer and the switchable optical layer.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 60/402,837 filed Aug. 12, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device comprising a liquidcrystal layer.

2. Brief Description of Prior Developments

FIG. 1 illustrates an existing transmissive liquid crystal display (LCD)device 10. The transmissive LCD device 10 comprises a front polariser12, a transparent mechanical substrate 14, a transparent unitaryelectrode 16, a nematic liquid crystal layer 18, a transparentpixellated electrode 20 comprising a plurality of distinct electrodeseach of which has an associated pixel switch 24, a transparentmechanical substrate 22, a rear polariser 26 and a back light 28.

The front polariser 12 and the rear polariser 26 are arranged ascross-polarisers. The mechanical substrate 14 supports the transparentunitary electrode 16 and provides mechanical rigidity to the device. Itis typically made of glass and supports the front polariser 12 on itsupper surface and the transparent unitary electrode 16 on its lowersurface. The mechanical substrate 22 has the plurality of pixel switches24 defined on its upper surface. Each of the pixel switches iselectrically connected to one of the plurality of electrodes of thetransparent pixellated electrode 20. The lower surface of the mechanicalsubstrate 22 supports the rear polariser 26. The nematic liquid crystallayer 18 is positioned between the transparent unitary electrode 16 andthe transparent pixellated electrode 20. When a pixel switch is on, avoltage is applied across the overlying portion of the liquid crystallayer 18. The back light is positioned beneath the rear polariser 26.Light from the back light 28 is initially polarised to a particularorientation by the rear polariser 26. As this light travels through thenematic liquid crystal layer 18 its polarisation will be changed againto either a first orientation or a second orientation depending uponwhether the portion of the liquid crystal through which it is travellinghas a voltage across it. The front polariser 12 will only let light passthrough having polarisation of the first orientation. Therefore, thetransmissive liquid crystal device 10 is selectively transmissive independence upon the state of the pixel switches 24. This type of devicehas good image quality in low and at normal ambient light conditions butit is very difficult to see in bright conditions. In addition, thedevice requires a permanent back light which uses too much power formany modern applications.

FIG. 2 illustrates an existing reflective liquid crystal display device30. The reflective LCD device 30 comprises a front light 38, a frontpolariser 12, a mechanical substrate 14, a transparent unitary electrode16, a nematic liquid crystal layer 18, a pixellated reflective electrode32 comprising a plurality of distinct reflective electrodes each ofwhich has an associated pixel switch 34 and a mechanical substrate 36.

The differences between the reflective LCD device 30 and thetransmissive LCD device 10 are that the reflective device 30 has a frontlight 38 positioned above the front polariser 12 and does not have aback light; the pixellated electrode 32 between the lower mechanicalsubstrate 36 and the liquid crystal layer 18 is reflective and is nottransparent; and the absence of a rear polariser in the reflective LCDdevice 30.

Light from the front light 38 passes through the front polariser 12 sothat it is polarised in a first orientation, when the light is reflectedby a pixellated electrode its polarisation is changed. The polarisationof the light is further changed by passing through the liquid crystallayer 18 and the extent of this change is dependent upon the voltageapplied across the portion of the liquid crystal layer through which thelight travels, which is in turn dependent upon whether the pixel switchassociated with that portion of the liquid crystal layer 18 is on oroff. Depending upon the extent to which the liquid crystal layer changesthe polarisation of the reflected light, the front polariser 12 eitherwill or will not pass the light. Thus the pixel switches 34 can be usedto make portions of the display either reflective or absorptive. Aproblem with this type of device is that the image quality of thedisplay is reduced by the presence of a front light 38.

Transflective liquid crystal display devices address this problem byremoving the front light 38 and introducing a back light. Each of thedistinct electrodes making up the pixellated reflective electrode layer32 is adapted to have a small hole in it which lets light through fromthe back light. However, transflective liquid crystal display devicesare a poor compromise between transmissive and reflective liquid crystaldisplays. If the display is optimised towards being bright in daylightby making the holes in the reflector smaller, then it will be darkerwith the back light on or will need more power in the back light to getthe same brightness.

It would be desirable to provide a display device which has good imagequality irrespective of the ambient light conditions and does not haveexcessive power requirements.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided adisplay device comprising: a first electrode; a liquid crystal layerpositioned under and connected to the first electrode; a secondelectrode; a switchable optical layer, having a transparent state and anon-transparent state and being electrically switchable between thetransparent state and the non-transparent state, positioned above andconnected to the second electrode; and a third electrode positionedbetween the liquid crystal layer and the switchable optical layer.

In one embodiment the non-transparent state of the switchable opticallayer is a reflective state. In another embodiment the non-transparentstate of the switchable optical layer is a light emissive state.

According to another aspect of the present invention there is provided amethod of controlling a display device comprising a liquid crystal layerand an underlying transparent/reflective layer which is either uniformlytransparent or uniformly reflective, to operate in a first mode by:selectively controlling portions of the liquid crystal layer anduniformly maintaining the transparent/reflective layer in a transparentstate, and to operate in a second mode by: selectively controllingportions of the liquid crystal layer and uniformly maintaining thetransparent/reflective layer in a reflective state.

According to a further aspect of the present invention there is provideda method of controlling a display device comprising a liquid crystallayer and an underlying transparent/emissive layer which is selectivelyeither transparent or emissive, to operate in a transmissive mode by:selectively controlling portions of the liquid crystal layer anduniformly maintaining the transparent/emissive layer in a transparentstate, and to operate in an emissive mode by: uniformly controlling theliquid crystal layer and selectively enabling portions of thetransparent/emissive layer so that the selected portions emit light.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to understandhow the same may be brought into effect reference will now be made byway example only to the following drawings, in which:

FIG. 1 illustrates a prior art transmissive liquid crystal displaydevice;

FIG. 2 illustrates a prior art reflective liquid crystal display device;

FIG. 3 illustrates a first embodiment of the present invention;

FIG. 4 illustrates a second embodiment of the present invention; and

FIG. 5 illustrates a mobile device having a display according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 illustrates a display device 100 which can operate as either atransmissive liquid crystal display device or as a reflective liquidcrystal display device. The display device 100 comprises a frontpolariser 12 supported on an upper surface of an upper mechanicalsubstrate 102, a plurality of pixel switches 106 defined on a lowersurface of the upper mechanical substrate 102, a pixellated electrode108 comprising a plurality of distinct electrodes each of which isassociated with a pixel switch 106, lies between the upper mechanicalsubstrate 106 and a nematic liquid crystal layer 18, a first unitarytransparent electrode 110 underlies the liquid crystal layer 18, aswitchable optical layer 112 underlies the first unitary transparentelectrode 110, a second unitary transparent electrode 113 underlies theswitchable optical layer 112 and is supported on an upper surface of atransparent lower mechanical substrate 114, and a rear polariser 26 issupported on the lower surface of the lower mechanical substrate 114. Aback light 28 is positioned underneath the rear polariser 26. The frontpolariser 12 and the rear polariser 26 are arranged as cross-polarisers.The upper mechanical substrate is transparent and is preferably made ofglass. Each of the pixel switches 106 is either a transistor, forexample formed from a amorphous silicon or a diode. The pixellatedtransparent electrode 108 is formed from a layer of indium tin oxide(ITO) and each of the plurality of separate electrodes which forms thepixellated transparent electrode 108 is typically a square of about 0.2mm in length.

The nematic liquid crystal layer 18 is a standard nematic liquid crystallayer and its polarisation properties are dependent upon the voltageapplied across it. The first unitary transparent electrode 110 is formedfrom a single layer of indium tin oxide. The switchable optical layer112 is typically tens of micrometers thick. It is preferably formed froma polymer dispersed liquid crystal (PDLC) but it can also be formed fromcholesteric liquid crystals. The important property of the switchableoptical layer 112 is that it can be switched from being in asubstantially transparent state to being in a substantially reflectivestate. The second unitary transparent electrode 113 is formed from asingle layer of indium tin oxide. The lower mechanical substrate istransparent and is preferably formed from glass.

The display device 100 also comprises a pixel controller 116 which isconnected to selectively enable the pixel switches 106 and atransmissive/reflective controller 118 which is connected to the secondunitary transparent electrode 112 and the back light 28. The firstunitary transparent electrode is connected to a constant first voltageV1. When the display device 100 is operating as a transmissive display,the transmissive/reflective controller 118 supplies a voltage V1 to thesecond unitary transparent electrode 113 and switches on the back light28. The first unitary transparent electrode 110 and the second unitarytransparent electrode 113 are at the same voltage and no voltage isdeveloped across the switchable optical layer 112. Consequently, theswitchable optical layer 112 is in its transparent state. (Inalternative embodiments, the optical layer may be reflective with novoltage across it and transparent with a voltage across it). The pixelcontroller 116 selectively switches on a first plurality of pixelswitches 106 and switches off a second plurality of pixel switchesthereby defines on the upper surface of the display device an image. Inthis mode of operation, the display device 100 operates in a similarmanner to the transmissive device 10 described in relation to FIG. 1.There are however significant structural differences. In particular, thepixellated electrode 108 and pixel switches 106 are positioned above theliquid crystal layer 18 and the unitary electrode is positioned beneaththe liquid crystal layer 18. In addition the unitary electrode isreplaced by a sandwich structure comprising a first unitary transparentelectrode 110, a switchable optical layer 112 and a second unitarytransparent electrode 113.

When the display device 100 is operating as a reflective display device,the transmissive/reflective controller 118 provides a voltage V2 to thesecond unitary transparent electrode 113 and does not switch on the backlight 28. The first unitary electrode 110 and the second unitaryelectrode 113 are at different voltages and a potential is developedacross the switchable optical layer 112. Consequently, the switchableoptical layer 112 becomes reflective (In alternative embodiments, theoptical layer may be reflective with no voltage across it andtransparent with a voltage across it). The pixel controller 116 is usedto define the image produced by the display device 100. In thisreflective mode of operation, the display device 100 operates in asimilar manner to the reflective LCD device 30 described in relation toFIG. 2. There are, however, some major structural differences. In thisembodiment, the pixellated electrode 108 and the pixel switches 106replace the unitary transparent electrode above the liquid crystal layer18 and must therefore be transparent. In addition, a sandwich structurecomprising the first unitary transparent electrode 110, the switchableoptical layer 112 and the second unitary transparent electrode 113, nowlies beneath the liquid crystal layer 18.

In the reflective mode, the display device 100 acts like a reflectiveLCD and has the image quality of a normal reflective LCD. In thetransparent mode, the display device 100 acts like a transmissive LCDand has the same image quality as a normal transmissive LCD. Thisembodiment therefore represents an optimum combination of reflective andtransmissive liquid crystal displays.

FIG. 4 illustrates a display device 120 according to a second embodimentof the present invention. The display device 120 can operate as areflective liquid crystal display device or as a light emissive device.The display device 120 comprises a front polariser 12 supported by theupper surface of a upper mechanical substrate 14, a unitary transparentelectrode 16 supported by the lower surface of the upper mechanicalsubstrate 14, a nematic liquid crystal layer 18 underlying the unitarytransparent electrode 16, a first pixellated transparent electrode 122underlying the liquid crystal layer 18, a switchable optical layer 124underlying the first pixellated transparent electrode 122, a secondpixellated transparent electrode 126 underlying the switchable opticallayer 124 and overlying a upper surface of a lower mechanical substrate128 on which has been defined a plurality of pixel switches 130.

The first pixellated transparent electrode 122 comprises a plurality ofdistinct electrodes. The second pixellated transparent electrode 126also comprises a plurality of distinct electrodes. Each one of thedistinct electrodes of the first pixellated transparent electrode 122overlies and opposes, with the switchable optical layer 124 intervening,an associated one of the distinct electrodes of the second pixellatedtransparent electrode 126. Each one of the distinct electrodes of thesecond pixellated transparent electrode 126 is associated with a pixelswitch 130.

The upper mechanical substrate 14 is transparent and preferably made ofglass. The unitary transparent electrode 16 is preferably made fromindium tin oxide (ITO) deposited on the lower surface of the uppermechanical substrate 14. The first pixellated transparent electrode 122is formed from a layer of indium tin oxide. The switchable optical layer124 has the physical property that it can be switched from beingsubstantially transparent to being light emissive. The switchableoptical layer 124 is preferably formed from organic light emittingdiodes (OLED)—light emitting dendrimers and inorganic electroluminescentlayers could also be used. The second pixellated reflective electrode126 is formed from a layer of metal deposited on the lower mechanicalsubstrate 128.

The unitary transparent electrode 16 is connected to a constant voltagesupply V3. A pixel reflective/emissive controller 132 controls whetheror not the display device 120 is operating as a reflective liquidcrystal display device or as a light emissive device. When the displaydevice 120 is operating as a reflective liquid crystal display device,each distinct electrode of the first pixellated transparent electrode122 is electrically connected in parallel with its associated distinctelectrode of the second pixellated reflective electrode 126. Thus, if apixel switch 130 is switched on at a particular pixel, the distinctelectrode of the second pixellated reflective electrode 126 overlyingthe pixel switch 130 will be brought to a first voltage and theoverlying associated distinct electrode of the first pixellatedtransparent electrode 122 will be brought to the same first voltage.Consequently, no voltage will be developed across any part of theswitchable optical layer 124. The whole of the switchable optical layer124 would therefore be transparent. However, a voltage will beselectively developed across portions of the liquid crystal layer 18 asthe pixel switches 130 are switched on or off.

The display device 120 therefore operates in a manner similar to thereflective LCD device described in relation to FIG. 2. However, thereare a number of significant differences. The display device 120 does nothave a front light, and a pixellated transparent electrode 122 andswitchable optical layer 124 lie in between the liquid crystal layer 18and the pixellated reflective electrode 126.

When the display device 120 is operating as a light emissive device, allof the distinct electrodes making up the first pixellated transparentelectrode 122 are electrically connected together and connected to theconstant voltage V3. The first pixellated transparent electrode 122 inthis electrical configuration operates as a unitary electrode and novoltage is developed across the liquid crystal layer 18. The pixelswitches 130 supply the voltage or current necessary to enable theoverlying portions of the switchable optical layer to emit light.Consequently, by the selective enablement of the pixel switches 130,selective portions of the switchable optical layer 124 will emit lightto produce an image on the display device 120.

FIG. 5 illustrates a mobile device 200 comprising a display device 100or 120. The mobile device 200 may be a hand-portable communicationsdevice such as a mobile phone or a personal digital assistant.

Although the present invention has been described in the precedingparagraphs with reference to various examples, it should be appreciatedthat modifications and variations to the examples given can be madewithout departing from the scope of the invention as claimed.

Whilst endeavouring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

1. A display device comprising: a first electrode; a liquid crystallayer positioned under and connected to the first electrode; a secondelectrode; a switchable optical layer, having in use either atransparent state or a non transparent state and being electricallyswitchable between the transparent state and the non transparent state,wherein the non transparent state is a reflective state and theswitchable optical layer is positioned above and connected to the secondelectrode; a third electrode positioned between the liquid crystal layerand the switchable optical layer, wherein the third electrode is sharedby the liquid crystal layer and the switchable optical layer; and abacklight positioned under the second electrode.
 2. A display device asclaimed in claim 1 wherein the first electrode is pixellated and hasassociated pixel switches.
 3. A display device as claimed in claim 1wherein the switchable optical layer is arranged to be switched as awhole.
 4. A display device as claimed in claim 1 wherein the firstelectrode is a pixellated transparent electrode comprising a pluralityof distinct electrodes.
 5. A display device as claimed in claim 4further comprising pixel switches positioned above the pixellatedtransparent electrode, such that each one of the pixel switches isconnected to one of the plurality of distinct electrodes.
 6. A displaydevice as claimed in claim 1 wherein the second and third electrodes areunitary electrodes.
 7. A display device as claimed in claim 1 whereinthe unitary electrodes are transparent.
 8. A display device as claimedin claim 1 further comprising a first polariser positioned above thefirst electrode, and a second polariser, crossed with the firstpolariser, positioned under the second electrode, and above thebacklight.
 9. A display device as claimed in claim 1 further comprisingcontrol means arranged to vary the voltage across the switchable opticallayer and to control the backlight.
 10. A display device as claimed inclaim 1 wherein the third electrode is transparent.
 11. A mobile devicecomprising a power supply and display device, as claimed in claim
 1. 12.A method of controlling a display device comprising a liquid crystallayer, an underlying switchable optical layer which is either uniformlytransparent or uniformly reflective, a first electrode positionedbetween and shared by the liquid crystal layer and the switchableoptical layer, to operate in a first mode by: selectively controllingportions of the liquid crystal layer and uniformly maintaining theswitchable optical layer in a transparent state using the firstelectrode, and to operate in a second mode by; selectively controllingportions of the liquid crystal layer and uniformly maintaining theswitchable optical layer in a reflective state using the firstelectrode.